1. Technical Field
The present invention relates to food products and to their methods of preparation. More particularly, the present invention relates to processed intermediate moisture vegetables of enhanced shelf stability and to infusion methods of their preparation.
2. Background Art
Dried packaged convenience foods or mixes are popular from which can be prepared foods such as soups, casseroles and salads. These packaged foods frequently contain dehydrated vegetables of various types along with various other dried ingredients especially pastas. Ideally, the vegetables would rehydrate quickly. Further, upon rehydration, the rehydrated vegetables would resemble as closely as possible fresh vegetables in terms of color, appearance, texture and nutrition.
Rehydrated green vegetables are especially vulnerable to exhibit color degradation compared to freshly blanched green vegetables. The color degradation is due in part to the deterioration of chlorophyll. Yellow vegetables in contrast are less susceptible to color loss.
Conventionally, vegetables are dehydrated using forced hot air convection drying. In the normal air drying of vegetables, approximately 75% of the moisture is removed during the first 25% of the drying time. In the final 75% of the drying time, a mere 20% of the original moisture is removed. In the final 75% of the drying time most of the flavor, texture and initial color loss occurs. Moreover, because of high energy input and slow throughput through capital intensive equipment such processes have proved to be expensive. With most common vegetables, the maximum moisture content after dehydration permissible consistent with color stability is approximately 5% or a water activity ("Aw") of about 0.4 but good commercial practice is for the moisture content to be considerably lower and this prolongs the time of processing of vegetables so treated. Thus, while further drying results in greater initial color loss, it reduces subsequent color loss upon storage and so provides greater overall color stability.
The terms "dry" and "dehydrated" are occasionally used informally or even interchangably in the art. For purposes of the present invention, the term "dehydrated" refers to vegetables having a moisture content of less than about 5% by weight and the verb "to dehydrate" means to dry to a dehydrated state. These terms are thus to be distinguished from "to dry" which is used herein merely to refer to some reduction in moisture content.
One straightforward technique to improve dried product qualities is to reduce drying times by reducing the size or thickness of the vegetable pieces. While this technique may be suitable for certain vegetables or parts, especially roots, e.g., carrots, this technique is undesirable to use with many vegetables or parts especially where a minimum size is needed in order to visually identify the vegetable, e.g., a broccoli floret. Another technique used to give the appearance of an improved product is to compensate for the natural color loss by the addition of dyes. Such dye addition is undesirable in view of regulatory and/or nutritional considerations. Still other techniques suggested for use for conventionally dried green vegetables is to reduce storage temperatures and to reduce exposure to oxygen. Each of these techniques improve color retention modestly.
For dry mixes containing dried vegetables of modestly superior quality in terms of color, flavor and texture upon rehydration, but still relying upon naturally occurring color it is common for manufacturers to use freeze-dried dehydrated vegetables. These however, are quite expensive, largely as a result of the high cost of capital equipment and high energy usage associated with their processing. In addition, because of their need to be maintained at a low moisture content, sophisticated packaging materials must be used for their storage and distribution.
Conventionally, air dried vegetables also can suffer from poor rehydration rates and poor texture upon rehydration. The prior art includes numerous improvements in conventionally air dried vegetables or fruits directed in part toward overcoming these two problems. For example, the prior art teaches infusion of a wide variety of solutes, especially salt and/or polyols for vegetables (see, for example, U.S. Pat. No. 4,361,589 issued Nov. 30, 1982 to Wauters et al. which discloses infusion of sugars from an alkaline solution to realize dried infused celery) and sugars or sugar alcohols for fruits (see, for example, U.S. Pat. No. 4,350,711 issued Sep. 21, 1982 to M. L. Kahn) have been introduced into the vegetables whereby the water activity of the dried product can be substantially reduced so that "intermediate moisture" but nonetheless shelf stable dried products can be prepared. The art also includes methods for finely controlling the amounts of solutes infused and for reduction of drying times (see for example, U.S. Pat. No. 4,447,460, issued May 8, 1984 to Lewis et al.) which involve pre-drying prior to application of carefully controlled addition of salt brine while the vegetables are still warm.
Sugar and sugar alcohols are, however, typically not used in connection with vegetables because of the undesirable sweetness they impart when used at levels effective to provide water activity control. The infused products, when boiled with water, or when boiling water is poured on to them, rehydrate more rapidly because of the presence of solutes which induce more rapid intake of water into the vegetable tissue. Unfortunately, while effective to improve the rehydration and texture problems of air dried vegetables, the infusion treatment aggravates initial color loss and can even accelerate subsequent color deterioration.
The food processing art also includes many teachings with respect to canned vegetable formulation and processing. While the objectives of providing a finished product as similar as possible to freshly cooked or blanched green vegetables are the same as those of the present invention, the teachings unfortunately are of little value. In canned products, vegetables have been subjected to retorting conditions to ensure stability and the vegetables exist in storage in an aqueous, sterile environment. In contrast, dried vegetables are not subjected to such temperatures and exist in a dehydrated environment. Because of these differences, the chemical pathways for color degradation are materially different and thus the teachings of the canning art have no reliable predictive value when applied to dried vegetable processing.
Notwithstanding these numerous art attempts at providing dried vegetables, to date, no shelf-stable dried green vegetable has been developed, or any method of preparing such a product, which not only possesses the desired texture and rehydration advantages of infused vegetables but also substantially retains its green color for extended periods when stored at room temperatures.
Accordingly, it is an object of the present invention to provide dried green vegetables with superior color compared to dried vegetables of comparable water activity when stored at room temperature.
It is another object of the present invention to provide dried green vegetables when superior color compared when stored at reduced or frozen temperatures compared to conventional dried vegetables.
Still another object of the present invention is to provide relatively large vegetable pieces with improved color retention and rehydrated texture properties.
Another object of the present invention is to provide dried, infused vegetables which nonetheless have color at least equal to air dried vegetables of equivalent water activity when stored at room temperature.
Another object of the present invention is to provide methods of preparing dried vegetables suitable for use with vegetable flowers, vegetable fruits and roots.
A still further object of the present invention is to provide methods of preparing vegetables which can be used with mixtures of green and yellow vegetables.
Another object of the present invention is to provide a simple, but reliable, process for the dehydration of vegetables wherein color retention is substantially improved.
It has been surprisingly discovered that the above objectives can be realized and dried vegetable products can be provided which are shelf stable and which nonetheless retain a very high portion of their green color for extended periods by infusing alkaline buffer systems and tocopherol along with conventional solutes in controlled amounts.